The purpose of the Air Traffic Control system at airports is to observe and control the movement of aircraft in the vicinity of the airport, both in the air and on the ground. The key factor in the successful operation of such a system is an individual, frequently called the Air Traffic Controller. This individual has responsibility for positioning of various aircraft; he/she communicates with the aircraft through a radio system. The Controller is assisted by various means; visual contact with the aircraft where feasible, radar contact, radio voice contact and other position indicators. The responsibility includes all observation of the target aircraft and all decisions concerning the movement of the aircraft and how this should be integrated with that of other aircraft and other physical objects in the vicinity of the airport.
It can be seen that the responsibilities of the Controller are extensive, and further that an error by the Controller can result in the loss of the aircraft and the death of many people, both aboard the aircraft as well as some of those on the ground. A recent example of such an error was the fatal accident in Los Angeles, when a Controller mistakenly directed two aircraft to use the same runway, one for landing an aircraft where the Controller had previously directed a different aircraft to use the same runway for taking off.